Structure and method for performance improvement in vertical bipolar transistors

ABSTRACT

A method of forming a semiconductor device having two different strains therein is provided. The method includes forming a strain in a first region with a first straining film, and forming a second strain in a second region with a second straining film. Either of the first or second strains may be either tensile or compressive. Additionally the strains may be formed at right angles to one another and may be additionally formed in the same region. In particular a vertical tensile strain may be formed in a base and collector region of an NPN bipolar transistor and a horizontal compressive strain may be formed in the extrinsic base region of the NPN bipolar transistor. A PNP bipolar transistor may be formed with a compression strain in the base and collector region in the vertical direction and a tensile strain in the extrinsic base region in the horizontal direction.

FIELD OF THE INVENTION

The invention relates to semiconductor devices, and more particularly tostrained films in semiconductor devices including bipolar transistorsand method of manufacture.

BACKGROUND DESCRIPTION

As semiconductor device applications are expanded, a need for deviceshaving a higher frequency response has arisen. For example, presentgeneration bipolar devices typically have power-gain cut off frequencies(f_(max)) of about 350 GigaHertz (GHz). Consequently, the maximumfrequency where bipolar devices provide a power gain greater than one isabout 350 GHz. The maximum current gain cut-off frequency (f_(T)) ofsuch devices is similarly limited to around 300 GHz. Furthermore, thef_(max) of a transistor is an important device parameter to maximize andis especially relevant to RF applications.

The power-gain cut-off frequency of a bipolar device is influenced by anumber of factors, particularly, the base resistance (R_(B)), thecapacitance between the collector and the base (C_(CB)), and its f_(T).The f_(T) is influenced by the transit time of carriers through emitter,base, and collector regions. Typically, the transit-time in the base andcollector regions dominate the overall carrier transit-time, and shouldbe minimized.

In order to maximize the f_(max), the product of R_(B) and C_(CB) may bereduced, and the f_(T) of the transistor may be increased. An increasein f_(T) can be achieved by reducing the base transit-time as well asthe collector transit-time. Traditional methods of improving the carriertransit time in the base layer include reducing the base layer thicknessand increasing the Ge-induced built-in electric field. Similarly, thecollector-base transit-time is traditionally addressed by reducing thethickness and resistance of the collector and by increasing thecollector doping concentration (N_(C)). However, increasing N_(C) alsoincreases the capacitance between the base and the collector and thusprovides only a marginal benefit to f_(max) improvement.

Accordingly, traditional dimensional scaling approaches to improve f_(T)or f_(max) can create other problems that reduce the benefits of suchscaling approaches. Consequently, further improvements to increasingcut-off frequencies may require non-traditional techniques. It is wellknown in the art that carrier mobility can be improved by inducingstrain (tensile or compressive) in a doped silicon material, therebypositively influencing the terminal characteristics of a device that isbuilt thereof. For example, a process-induced tensile strain in thechannel of an nFET can create improved electron mobility leading tohigher saturation currents. In such situations, a tensile strain in thechannel may be induced by applying a compressively strained nitride filmclose to the active region of the FET.

SUMMARY OF THE INVENTION

In a first aspect of the invention, a method of fabricating asemiconductor, includes doping a region of a structure, and forming afirst film on a first portion of the structure to induce a first strainin the first portion. The method also includes forming a second film ona second portion of the structure to induce a second strain in thesecond portion.

In another aspect of the invention, a method of straining asemiconductor device, includes forming a first straining film on asidewall of a structure inducing a first strain in an adjoining dopedregion, and forming a second straining film on a top portion of thestructure to induce a second strain in an adjoining region, where thefirst strain is different from the second strain.

In another aspect of the invention, a semiconductor device includes acollector region, a base region formed on a collector region, and anemitter region formed on the base region. The semiconductor device alsoincludes a first straining film inducing a first strain in an adjoiningregion formed on a side of the collector region and the base region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates strain in a semiconductor device;

FIG. 2 illustrates strain in a semiconductor device;

FIG. 3 illustrates a combination of the strain components for asemiconductor device;

FIG. 4 illustrates a combination of the strain components for asemiconductor device;

FIG. 5 illustrates a step of an embodiment of forming a bipolartransistor in accordance with the invention;

FIG. 6 illustrates a step of an embodiment of forming a bipolartransistor in accordance with the invention;

FIG. 7 illustrates a step of an embodiment of forming a bipolartransistor in accordance with the invention;

FIG. 8 illustrates a step of an embodiment of forming a bipolartransistor in accordance with the invention;

FIG. 9 illustrates a step of an embodiment of forming a bipolartransistor in accordance with the invention; and

FIG. 10 illustrates a step of an embodiment of forming a bipolartransistor in accordance with the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention is directed to creating high speed semiconductor devices,and in particular for creating bipolar devices capable of approachingTeraHertz (THz) frequency operation. Embodiments of the invention allowsuch high frequency response possibilities by utilizing strained filmsthat induce strain in doped semiconductor regions which enable thereduction of parasitic resistance and transit times within certainregions of the device without increasing unwanted side-effects. Itshould be noted that stress and strain, and stressing and straining areproportional to one another based on Young's Modulus, and the terms maybe used interchangeably herein. However, for simplicity, the term“strain” will be used throughout. Embodiments of the invention combinedifferent films inducing either different types of strain in asemiconductor region and/or different orientations of strain in a regionadjacent to the strain film that positively influences the carriermobility in the device. For example, for a NPN bipolar device, a tensilestrain can be induced in the base and the collector, whilesimultaneously inducing a compressive strain in the extrinsic baseregion of the device. Accordingly, simultaneous improvement in collectortransit time, collector-base transit time, cut-off frequency for currentgain and resistance in the base are realized without an increase inunwanted parasitic effects.

Referring to FIG. 1, a schematic of a semiconductor device is shownhaving a collector 10 with a base 15 formed thereon. An emitter 20 isformed on top of the base 15. Also shown is an applied compressivestrain 25 being applied to sidewalls of the collector 10, base 15 andemitter 20. The applied compressive strain 25 applied to the sides ofthe device induces an induced tensile strain 30 within the collector 10,base 15 and emitter 20, in the vertical direction as shown. The inducedtensile strain 30 is generally at a right angle to applied compressivestrain 25. Accordingly, the induced tensile strain 30 causes low fieldelectron mobility enhancement in a vertical direction, which improvesbase 15, and collector 10 transit times. Improving the base 15 andcollector 15 transit times cause improvements in maximum current gaincutoff frequency f_(T) with reduced unwanted side effects.

In other words, FIG. 1 shows an external applied compressive strain 25applied at the sidewall surface induces an induced tensile strain 30 inthe semiconductor lattice along the vertical direction. The inducedtensile strain 30 spreads from the sidewall edges of the semiconductorto the mid region, where the active region of a bipolar may be formed.In the case of an n-doped material, this induced tensile strain 30 willhelp in improving the electron mobility and reducing the resistance aswell as the transit-time.

Referring to FIG. 2, a schematic of a semiconductor device having acollector 10, base 15, and emitter 20 similar to FIG. 1 is shown. Thebase 15 and emitter 20 have a film with tensile strain 35 applied ontheir surface. The film with tensile strain 35 induces a compressivestrain 40 within the base 15 and emitter 20, in the horizontal directionas shown. The induced compressive strain 40 is oriented in the samedirection as the applied tensile strain 35. The induced compressivestrain 40 may be located in the extrinsic region of the base 15 (outsidethe regions of the base 15 rather than under the emitter region 15).Consequently, the induced compressive strain 40 enhances low field holemobility in the horizontal direction which improves base 15 resistanceand improves the maximum frequency f_(max) of the device, reducedunwanted side effects.

In other words, FIG. 2 shows an external film with tensile strain 35applied at a top surface of a semiconductor device which induces acompressive strain 40 in the semiconductor lattice along a horizontaldirection. In the case of an p-doped material, this induced compressivestrain 40 will help in improving the hole mobility and reducing theresistance.

Referring to FIG. 3, a schematic of a semiconductor device having acollector 10, base 15, and emitter 20 similar to FIG. 1 is shown. Thecollector 10, base 15, and emitter 20 have an applied compressive strain25 applied to the sides thereof. The applied compressive strain 25induces tensile strain 30 in the semiconductor at right angles to theapplied compressive strain 25 within the collector 10, base 15, andemitter 20. A film with tensile strain 35 is also applied along the topof the base 15. The applied film with tensile strain 35 inducescompressive strain 40 in the semiconductor along the top of the base 15.Consequently, an induced tensile strain through the collector 10, base15, and emitter 20 can be created simultaneously with an inducedcompressive strain 40 through the top of the base 15. The inducedtensile strain 30 is substantially at a right angle to the inducedcompressive strain 40. In other words, the induced tensile strain 30 isin a vertical direction, and the induced compressive strain 40 is in ahorizontal direction.

In other words, FIG. 3 shows improving electrical performance of asemiconductor device by an appropriate application of strain componentsto the semiconductor device, such as, for example, a vertical NPNbipolar transistor in which hole transport is substantially in ahorizontal direction and electron transport is in a vertical direction.

Referring to FIG. 4, a collector 10, base 15, and emitter 20 are shown,similar to FIG. 1. The collector 10, base 15, and emitter 20 have anapplied tensile strain 27 applied to their sides. The applied tensilestrain 27 induces a compressive strain 33 within the collector 10, base15, and emitter 20. The induced compressive strain 33 is substantiallyat a right angle to the applied tensile strain 27. Additionally, acompressive strain 37 is applied near the top of the base 15, to inducea horizontal tensile strain in the base region 15.

The applied film with compressive strain 37 induces a tensile strain 43along a top of the base 15. Accordingly, an induced compressive strain33 and an induced tensile strain 43 are simultaneously created insidethe collection 10, base 15, and emitter 20 assembly. The inducedcompressive strain 33 is generally at a right angle to the inducedtensile strain 43. Additionally, the induced compressive strain 33 isgenerally in a vertical direction, while the induced tensile strain 43is generally in a horizontal direction.

One method of inducing a strain within regions of a semiconductor deviceincludes forming an applied strain on a surface of a region of thesemiconductor device by application of a straining film. Typicalthicknesses of such straining films, whether compressive or tensile, mayrange from about 10 nm to about 200 nm, and more typically between about40 nm to about 60 nm. After application, the applied straining film isallowed to relax thereby inducing an opposite type of strain that islocated within the region of the semiconductor device that is contactedby the applied film.

Accordingly, FIG. 4 shows improving electrical performance by anappropriate application of strain components for a semiconductor device,such as, for example, a vertical PNP bipolar transistor, in which theelectron transport is substantially horizontal and the hole transport issubstantially vertical.

Referring to FIG. 5, a starting structure is shown having a collector115 formed on a heavily doped subcollector region 110 that is formed ontop of a silicon substrate (not shown). The collector 115 is eitherpatterned or has a blanket doping like an N-type doping such as, forexample, arsenic or phosphorus. Also, positioned on top of thesubcollector 110 and surrounding the collector 115 is a very lightlydoped or neutral region 120. A base 125 is grown on top of the lowerneutral doped region 120 using an epitaxial process. The base 125 can beformed by any of the epitaxial processes well known in the art.

The base 125 receives a P-type doping such as, for example, boron orindium. The base 125 can be made of a compound semiconductor, such as,silicon-germanium or silicon-germanium-carbon, using well knowntechniques in the art. Additionally, on top of the base 125 is an upperemitter cap region 130, which may remain undoped or lightly doped. Theemitter cap region 130 can be formed by a low temperature epitaxialprocess.

Referring to FIG. 6, a strain film 135 is formed on a top of the emittercap region 130. In one embodiment, the strain film 135 may sit directlyon top of a thin oxide region 133 that is added above the emitter capregion 130. The strain film 135 is typically less than 1000 angstroms(Å) thick; however, other film thicknesses are contemplated for use bythe invention. The strain film may include any kind of film or layerwhich causes a strain to arise in the region adjacent to the strainfilm. In one embodiment, the strain film may be a film applied having aninternal first strain during application where the internal strain isallowed to relax after the film is applied thereby imparting an inducedstrain to the underlying material on which the strain film is deposited.For example, a tensile strain film is applied and allowed to relax,squeezing the adjoining substrate causing compressive strain in thesubstrate, i.e., a tensile film induces a compressive strain in thesilicon substrate.

The strain film 135 may be formed from a nitride, where the compositionof the nitride is suitably altered to produce a film having a straintherein. In one embodiment the strain film 135 provides a horizontalstrain component. It should be noted that the strain induced by thestrain film 135 may be in either the horizontal or the verticaldirection. It should also be noted that the strain in the strain film135 may be either compressive or tensile. After the strain film 135 isformed, outlying portions of the strain film 135, upper doped region130, base 125, and lower neutral doped region 120 are etched using knownprocesses such as RIE.

Referring to FIG. 7, an emitter oxide 145 is formed over the strain film135. The emitter oxide 145 may be formed by any process suitable fordepositing an oxide. The emitter oxide 145 is patterned afterdeposition. After the emitter oxide 145 is patterned, a hole is etchedthrough the emitter oxide 145 and the strain film 135 to a top of theupper neutral doped region 130. An emitter hole 140 is formed by any ofthe processes well known in the art for forming an emitter. In anotherembodiment, an emitter hole may be patterned directly on top of thestrain film 135, without the presence of the oxide film 145.

After the emitter 140 is formed, a polysilicon film 150 is formed overthe emitter 140. The polysilicon 150 may be formed by any of the methodswell known in the art for depositing polysilicon. The depositedpolysilicon 150 may be an in-situ doped using dopant such as, Arsenic orPhosphorus for n-type and Boron for p-type, and may be realigned to theexposed silicon region on 130 and within the emitter hole 140. Suchtechniques are well known in the art. The emitter poly is patterned andetched using standard techniques know in the industry. The poly/oxidestack or the poly film is etched and stopped on strain film 135. Astrain film 155 is then conformally deposited over the structureincluding the polysilicon 150, the strain film 135, the sides of theemitter cap region 130, the base 125, and the lower neutral doped region120. In one embodiment, the externally applied strain film 155 induces avertical strain component in the semiconductor.

Additionally, embodiments may include the strain film 155 as acompressive applied strain film when the strain film 135 is a tensilestrain film used in NPN devices. Alternatively, the strain film 155 maybe a tensile strain film when the strain film 155 is used in PNPdevices. In one example, an NPN structure will use a compressive strainfilm in the silicon mesa sidewall regions and a tensile strain film inthe extrinsic base regions. Thus, in these embodiments: i) low fieldelectron mobility enhancement is provided in the vertical direction canbe provided for improved base and collector transit times (utilizingcompressive strain film in the base and collector region); and ii) lowfield hole mobility enhancement is provided in the horizontal directionfor improved base resistance (utilizing tensile strain film in theextrinsic base region). For example, this is shown in FIGS. 1, 2, and 3,respectively.

Referring to FIG. 8, an oxide layer is deposited over the verticalstrain film 155 by any of the methods well know in the art fordepositing an oxide film and is then isotropically etched to leave anoxide spacer 180 on a lower portion of the vertical strain film 155 andan upper oxide shoulder 160 on an upper portion of the vertical strainfilm 155. Once the lower oxide shoulder 180 and the upper oxide shoulder160 have been formed, exposed portions of the vertical strain film 155are removed using any of the etching process well known in the art forremoving such a film.

By depositing and removing preselected portions of the vertical strainfilm 155, a lower vertical strain portion 165 remains on the sides ofthe lower neutral doped region 120, base 125 and upper emitter capregion 130. An upper vertical strain film 170 is also formed on a sideof the emitter 140, polysilicon 150, emitter shoulder oxide 145, andstrain film 135. Additionally, portions of the upper emitter cap region130 are exposed at the foot of the strain film 135. For example, wherean NPN device is being formed, an extrinsic base implant 185 isimplanted selectively in the exposed portions of the emitter cap region130, possibly using a mask step. The extrinsic base region 185 may beformed by implanting Boron (for NPN) dopant with a dose of 1e15 to 1e16cm⁻² at an energy of 5-15 keV.

Referring to FIG. 9, silicide is deposited on the subcollector region110, the extrinsic base implant 185, and the NP polysilicon 150 to forma collector contact 200, a base contact 195, and an emitter contact 190,respectively. The suicide deposition is a self-aligned salicidationdeposition using a salicidation deposition process well known in theart.

Referring to FIG. 10, a first dielectric material 205, such as, forexample, a nitride material is conformally deposited over the structure.This material is typically used for etch stopping the contacts to thesilicided regions, and is well known in the art. Next, the entirestructure is covered with a second dielectric material 207 such as, forexample, boron phosphate silicate glass, and later planarized using wellknown techniques in the field such as chemical-mechanical polishing(CMP). Via holes are then etched through the second dielectric 207 andthe first dielectric material 205. The via holes are then filled with aconductor, such as, tungsten. In particular, a collector via 210 isformed through the first dielectric material 205 and the seconddielectric material 207 to the collector contact 200. A base via 220 isformed through the first dielectric material 205 and the seconddielectric 207 to the base contact 195. Additionally, an emitter via 215is formed through the first dielectric 205 and the second dielectricmaterial 207 to the emitter contact 190.

As discussed above, a semiconductor device, such as, for example, abipolar device may be formed having two different types of strainingfilms applied thereon. The two different types of straining films areapplied to selected areas to cause a pre-selected strain in therespective adjoining areas. It should be noted that either of thestraining films may be configured to apply either one of a compressionor tension strain or a combination thereof. Additionally, either one ofthe straining films may be configured to form a strain in a vertical orhorizontal direction or virtually any other orientation.

Each of the straining films can be configured and located to strain aparticular region of a semiconductor device without straining otherregions of the semiconductor device. Consequently, the straining filmscan be arranged on, for example, an NPN bipolar transistor so that thebase and emitter regions receive a tensile strain in the verticaldirection, and the extrinsic base region receives a compression strainin the horizontal direction.

Additionally, another example of a semiconductor device which may beformed with the multiple straining films includes, for example, a PNPbipolar transistor where a compressive strain is induced in the base andcollector in a vertical direction by one of the straining films and atensile strain is induced in the horizontal direction in the extrinsicbase region by a second straining film. It should be noted that, ingeneral, these two forces may be oriented at right angles to one anotherand may coexist in the same region due to the right angle orientation ofeach strain.

It should also be noted that more than two different strains may beapplied to the semiconductor device using the methods discussed above.For example, a third strain film may be applied to a particular area ofthe device to provide a third strain to a selected region of the device.The strain can be either tensile or compressive, and may be oriented invirtually any direction.

While the invention has been described in terms of exemplaryembodiments, those skilled in the art will recognize that the inventioncan be practiced with modifications and in the spirit and scope of theappended claims.

1. A semiconductor device, comprising: a collector region; a base regionformed on a collector region; an emitter region formed on the baseregion; a first straining film providing a first strain in an adjoiningregion formed on a side of the collector region and the base region; anda second straining film, formed at least partially on the firststraining film, providing a second strain in an adjoining region formedon the top of the base region and next to a side of the emitter region,where the first strain is different from the second strain, the firststrain is a tensile strain in a substantially vertical direction and thesecond strain is a compressive strain in a substantially horizontaldirection.
 2. A semiconductor device, comprising: a collector region; abase region formed on a collector region; an emitter region formed onthe base region; a first straining film providing a first strain in anadjoining region formed on a side of the collector region and the baseregion; and a second straining film, formed at least partially on thefirst straining film, providing a second strain in an adjoining regionformed on the top of the base region and next to a side of the emitterregion, where the first strain is different from the second strain, thefirst strain is a compressive strain in a substantially verticaldirection and the second strain is a tensile strain in a substantiallyhorizontal direction.
 3. A semiconductor device, comprising: a collectorregion; a base region formed on a collector region; an emitter regionformed on the base region; a first conformal straining film, formedentirely over the base region, providing a first strain in an adjoiningregion formed on a side of the collector region and the base region; anda second straining film providing a second strain in an adjoining regionformed on the top of the base region and next to a side of the emitterregion, wherein the semiconductor device comprises an NPN transistor andthe first strain comprises a tensile strain in the collector and thebase, and the second strain comprises a compressive strain in the base.4. A semiconductor device, comprising: a collector region; a base regionformed on a collector region; an emitter region formed on the baseregion; a first conformal straining film, formed entirely over the baseregion, providing a first strain in an adjoining region formed on a sideof the collector region and the base region; and a second straining filmproviding a second strain in an adjoining region formed on the top ofthe base region and next to a side of the emitter region, wherein thesemiconductor device comprises an PNP transistor and the first strain isa compressive strain in the collector region and an extrinsic region ofthe base region, and the second strain is a tensile strain in theextrinsic region of the base region.
 5. A semiconductor device,comprising: a collector region; a base region formed on a collectorregion; an emitter region formed on the base region; a first conformalstraining film providing a first strain in an adjoining region formed ona side of the collector region and the base region; a neutral regionsurrounding the collector region; wherein the base region is on top ofthe neutral doped region; an emitter cap on top of the base region; anda strain film on a top of the emitter cap, wherein the strain film sitsdirectly on top of an oxide region above the emitter cap.
 6. Thesemiconductor device of claim 5, wherein the strain film is allowed torelax after the strain film is applied.
 7. A semiconductor device,comprising: a collector region; a base region formed on a collectorregion; an emitter region formed on the base region; a first conformalstraining film providing a first strain in an adjoining region formed ona side of the collector region and the base region; a neutral regionsurrounding the collector region; wherein the base region is on top ofthe neutral doped region; an emitter cap on top of the base region; astrain film on a top of the emitter cap; and an emitter oxide formedover the strain film and a polysilicon film formed over the emitter. 8.The semiconductor device of claim 7, wherein the first conformalstraining film is over the polysilicon film.
 9. The semiconductor deviceof claim 8, further comprising an oxide spacer on a lower portion of thefirst conformal straining film.
 10. A semiconductor device, comprising:a collector region; a base region formed on a collector region; anemitter region formed on the base region; a first conformal strainingfilm, formed entirely over the base region, providing a first strain inan adjoining region formed on a side of the collector region and thebase region; a neutral doped region surrounding the collector region,wherein the base region is on top of the neutral doped region; and anemitter cap on top of the base region, wherein portions of the emittercap above the collector are devoid of the first conformal strainingfilm.